A sensor network is a technology which has rapidly been attracting attention in recent years. By applying sensor network technology, sensing of information relating to the real world is performed in semi-real time, and this can be incorporated into an IT system as electronic information. Various applications are now being devised, and are expected to become next-generation IT techniques to support the coming ubiquitous society.
A key device in a sensor network is a wireless sensor node characterized by its compactness and low power consumption. Since it is compact, it can be attached to all manner of things including the environment and human beings, and since it is low-power, it can be operated for several years by battery without an external power supply. Further, since it communicates by wireless, it can be widely distributed over a wide area in combination with base stations or relay nodes.
A characteristic function of the sensor node is intermittent operation. This means that the only required hardware portion is driven when tasks such as sensing or data transmission are to be performed, and when there are no tasks to be performed, peripheral hardware such as sensors and RF are completely shut down, and a microprocessor is also put on standby in a low-power mode. By performing intermittent operation, prolonged operation of the sensor node is attained with limited battery resources.
In a sensor node which performs the aforesaid intermittent operation, the more the low-power consumption performance improves, the more the proportion of sleep time increases, so the sensor can receive data only during a very short waking time between sleep periods. How to transmit data efficiently in a compact, low-power sensor node which performs intermittent operation has an important effect on the performance of the system. In regard to this, a technique is disclosed where, for example, a sleep timing is controlled by a transmitting terminal which notifies a receiving terminal of the estimated time when data is to be transmitted on the next occasion (e.g., JP-A 2004-3 20153).